Protests involving thousands of people today inside and outside the Bella conference centre hosting UN climate talks in Copenhagen have passed off without major incident.

About 4,000 campaigners marched on the Bella centre in an attempt to stage a "people's assembly" inside the summit, but were stopped by police. At least 260 protesters have been detained following clashes outside the Bella centre, said the Danish police spokesman, Per Larsen.

Tim Jones, policy officer for anti-poverty campaigners the World Development Movement, said: "Today thousands of people sought to create a people's assembly to get voices heard offering real solutions to the climate crisis. The people's assembly was stopped by police who committed unprovoked violence on both protesters and official delegates to the UN negotiations. This is a moment in history where the right to protest is of vital importance."

Campaigners complained at officers' use of pepper spray to keep the march – organised by the Climate Justice Action and Climate Justice Now! (CJN) networks – away from the conference centre. Magnus Leitt, an eyewitness, said of the tactic: "[The spray] went right into my eyes, I couldn't do anything to get out of the way. It burns like fire. But the worst thing is that you start to hyperventilate, you can't get your breath and then it's hard not to panic," he said.

Patrick Gillett, a demonstrator who was arrested this morning, spoke to the Guardian from one of the cages at a holding centre in the city's Valby district. "We were coming down the cycle path towards the Bella centre and the police came to get us. They chased us with dogs and one bloke had to climb up a tree because the dog was biting his legs. Then they brought us here, with our hands cuffed behind our backs and we had to sit on the floor until they put us into the cages. There are about 10 people in my cage but some people have got 12, 20, 25."

Activists tried a variety of methods to enter the conference centre, approaching in large groups from several directions and, at one point, sending several hundred people running with seven giant lilos to bridge a moat next to the centre. One of protesters, Imke Eichelberg, said: "I think we've shown the politicians that they need to listen to us. There are a lot of people here and we want actions not words. I was pretty anxious about what the police were going to do but it's been OK."

Meanwhile, activists inside the conference centre staged a noisy walkout in protest at the lack of progress inside the negotiating rooms. Carrying banners reading "Reclaim power" and "Climate Justice Now", a group of about 60 NGOs marched past country delegates and corporate booths. Tom Goldtooth, a Bolivian with the Indigenous Environmental Network, led the chanting and drumming. "We feel they are negotiating for the sake of corporations," he said.

Wahu Kaara, a Kenyan member of the People's Movement for Climate Change, said direct action was vital after the failure of the talks so far. "Sovereignty of the people is the only solution to the climate crisis. Negotiators in the rooms make deals for profit at the expense of people's lives."

Yvo de Boer, the head of the UN's climate change secretariat, talked to the protesters inside the centre. "I'm stuck between a rock and hard place," he said, shortly before asking: "Do you want to talk to me or do you want to fight me?"

The protesters from inside the centre were blocked on a narrow bridge as they tried to link up with the main "people's assembly" demonstration outside. After initially promising they would be allowed to cross, the police drove them back, beating many of those in the front repeatedly with small clubs. "Shame on you!" the protesters chanted. "We care for your future more than you do."

Naomi Klein, who was among those who joined the walkout, said the Danish police's handling of the protests to date had been very poor. "Denmark is losing its reputation for being a good world citizen," she said.

Earlier in the day, several major accredited environmental groups, including Friends of the Earth, Avaaz and TckTckTck, were refused entry to the conference. One of the people denied access to the summit was Stuart Eizenstat, the US chief negotiator at Kyoto.